The result of reaction between a liquid sample and one or more reagent, preferably dry, depends on the quantity of the one or more reagent and the volume of liquid sample. Although any type of liquid sample is implied, serum, plasma and blood (also referred to as whole blood) are samples of particular interest. When blood is allowed to clot and the sample is centrifuged, the yellow liquid that sits on top of the blood clot is called serum. If the blood is collected in a tube containing an anticoagulant, for example heparin, and the sample is centrifuged, the yellow liquid that sits on top of the packed red blood cells is called plasma. The packed red cell volume (PCV) or hematocrit determines the percentage of red blood cells (RBCs) in whole blood. Since only the RBCs contain hemoglobin, total hemoglobin is highly correlated with hematocrit, except in cases of for example, macrocytic anemia where the mean red cell hemoglobin concentration is lower than that of a normal red cell. Some analyzers measure hematocrit by electrical conductivity, and convert the hematocrit measurement to a total hemoglobin concentration, and some analyzers measure total hemoglobin concentration by spectroscopy, and convert the total hemoglobin concentration to a hematocrit value. Spectroscopic calibration algorithms can be developed to measure both hematocrit and total hemoglobin concentration.
Point-of-care Testing (POCT) is defined as medical diagnostic testing performed outside the clinical laboratory in close proximity to where the patient is receiving care. POCT is typically performed by non-laboratory personnel and the results are used for clinical decision making. For the sake of convenience and rapid turnaround time, blood is the sample of choice. Due to the complexity of blood, certain tests can only be performed on serum or plasma.
POCT has a range of complexity and procedures that vary from manual procedures to automated procedures conducted by portable analyzers. POCT is most efficient when the sample of interest can be applied to or loaded onto a test cartridge, the sample inlet capped, and the remaining steps are performed automatically after the loaded test cartridge is inserted into a slot or receptor of an analyzer. Some blood tests, for example coagulation assays and immunoassays require a fixed volume of sample, for example, to ensure that when mixed with a reagent the ratio of the volume of sample to the volume of the reagent is held constant. Other tests, for example that determine electrolytes, do not require a fixed volume of sample. In the case of electrolytes, sample volume may not be an issue if the electrolyte concentration is estimated by measuring electrical activity in the sample.
Applying an unmetered sample volume to test strips is well known; some test strips contain absorbing sections that can accommodate a known volume of plasma, after the red cells are retained in another section of the test strip near the blood application site. In some cases, the hematocrit affects the plasma flow in test strips, and therefore correction for hematocrit may improve accuracy of the analyte measurement. In some systems, a pipette is used that is designed to aspirate a predetermined sample volume.
U.S. Pat. No. 6,750,053 to Opalsky et al and U.S. Pat. No. 7,682,833 to Miller et al disclose devices for rapidly metering samples. U.S. Pat. No. 6,750,053 describes a snap-shut seal and states (column 11 lines 16-19) that the “volume of the metered fluid sample is the volume of the holding chamber 20 between the orifice (48 in FIG. 5) in the wall of the holding chamber and the capillary stop 22.” U.S. Pat. No. 7,682,833 discloses (column 23 lines 39-43) that the “location at which air enters the sample chamber (gasket hole 27) from the bladder, and the capillary stop 25, together define a predetermined volume of the sample chamber. An amount of the sample corresponding to this volume is displaced into the first conduit when paddle 6 is depressed.” In the cases of U.S. Pat. No. 6,750,053 and U.S. Pat. No. 7,682,833, while the fluid sample is metered, the sample in the sample collection well (illustrated in U.S. Pat. No. 6,750,053 as element 12 in FIG. 3) is wasted.
Sample size is a major consideration for POCT systems, especially when it is desirable to use a small drop of blood obtained by puncturing the skin of a body part; the sample is referred to as a pin-prick sample. With some patients, it is difficult to obtain a small drop of blood, therefore there is a need to avoid any blood wastage. This is particularly true for neonatal blood testing.
Prothrombin Time (PT) is an example of a coagulation test, which requires a fixed sample volume. PT is usually reported as PT-INR (PT-International Normalized Ratio). The result for a prothrombin time performed on a normal individual will vary according to variations between different types and batches of thromboplastins used. The INR was devised to standardize the results using an ISI (International Sensitivity Index) value. Each manufacturer assigns an ISI value for any thromboplastin they manufacture, Another factor which affects PT-INR when using whole blood, as is the case of POCT, is the hematocrit. Only plasma contains coagulation factors, but a whole blood sample has a variable number of red cells mixed in, depending on the patient's hematocrit. These red cells take up space in the test cartridge. The coagulation factors that are being tested, are all in the liquid part of blood, i.e., the plasma. Because patients have different hematocrits, each patient sample adds a different amount of liquid plasma to the cartridge, but the amount of thromboplastin in the test cartridge is fixed. In a patient with low hematocrit, the excess plasma volume dilutes the reagent i.e., thromboplastin, and slows clot formation, resulting in a falsely increased PT-INR. PT-INR measured in the laboratory usually uses plasma, and plasma measurement of PT-INR is considered the gold standard. Therefore, whole blood PT-INR measurement will differ from the laboratory PT-INR measurement, which uses plasma. For POCT of PT-INR, correction can be made for an average hematocrit value, but errors in the PT-INR will increase as the hematocrit value moves away from the average hematocrit value. POCT of PT-INR usually use biosensors (also referred to as electrochemical detectors) that in many cases do not provide hematocrit measurement because the blood clots within seconds, after the blood is mixed with the thromboplastin.